Low-viscous, radiation curable formulation, particularly for the stereo-lithographical production of earpiece

ABSTRACT

The invention relates to a biocompatible, low-viscous, radiation curab]e formulation, particularly for use in stereolithography, for use in medical technology, in particular, for producing earpieces, containing: a) 55-95 % by weight of a monomeric or oligomeric dimethacrylate based on bisphenol A or bisphenol F; b) 0-20 % by weight of a urethane methacrylate having a functionality of n&lt;4 and a viscosity&lt;15 Pa s; c) 2-15 % by weight of a monomeric aliphatic or cycloaliphatic dimethacrylate having a viscosity&lt;5 Pa s; d) 0-15 % by weight of a monofunctional methacrylate having a viscosity&lt;3 Pa s; e) 0.5-6 % by weight of a photoinitiator or a combination of a number of photoinitiators whose absorption is located within the wavelength range of the laser beam used; f) 0.0001-2 % by weight of the inhibitor 2,2,6,6-tetramethylpiperidin-1-yloxy (free radical), also in conjunction with known inhibitors; g) 0-40 % by weight of fillers; h) 0-5 % by weight of coloring pigments; i) 0-5 % by weight of remaining additives such as UV stabilizers or spreading additives, whereby the proportions of constituents a) to h) total 100 % by weight.

The present invention relates to a low viscosity, radiation curableformulation, especially for the stereolithographic production ofproducts suitable for medicinal technology, especially for theproduction of earpieces, on a basis of at least two compounds which haveradical-polymerizable methacrylate functions radial and at least onephoto initiator suitable for the polymerization of the respectivecompounds and at least one source of an inhibitor in the form of the2,2,6,6-tetramethylpiperidine-1-yloxy (free radical) suitable forstabilizing the stereolithographic resin and which apart from otherinhibitors already known in the art of stereolithographic resins hasbeen found to be suitable as a highly advantageous formulation componentfor such compositions.

From U.S. Pat. No. 4,575,330, it is known that low viscosity, radiationcurable resins or resin mixtures can be used for the production of threedimensional objects by means of stereolithography.

Further, it is known from U.S. Pat. No. 5,487,012 and WP 01/87001 thatthe stereolithograpy can be advantageously employed for producingearpieces. In the stereolithographic process, three dimensional objectsare formed from a low viscosity, radiation curable formulation in that athin layer of the formulation (about 0.0025 to 0.01 mm in thickness) ofthe formulation can be so prehardened by actinic radiation in a definedmanner that the layer produced has the desired cross sectional shape ofthe object at the particular location. At the same time, the layerproduced in the hardening step is polymerized. The configuration of theobject can be defined with the aid of a computer controlled laser systemutilizing for example a Nd:YVO solid body laser (Viper Si² SLA System asmanufactured by the U.S. Firm 3D Systems). The shaped bodies thusgenerated can optionally be after-hardened, for example, by radiationcuring.

In the stereolithographic process, resin formations are used whichsatisfy special requirements. Thus mention can be made especially of theradiation sensitivity and the viscosity of the resin formulation as wellas the hardness of the shaped body prehardened by means of laserhardening. This incompletely hardened shaped body is referred to in thestereolithographic arts as a green body and the hardness of this greenbody is characterized by its modulus of elasticity (E-modulus) and itsbending strength, referred to as the green strength of the body.

The green strength is an important parameter in stereolithographicpractice since a shaped body with low green strength can deform duringthe stereolithographic process under its own weight or during theafter-hardening under a xexon arc lamp or halogen lamp can sag ordeform. It will thus be understood that because of the aforedescribedrequirements, complicated formulations and compositions have beendeveloped.

For example, H. Kodama in Rev. Sci. Instrum. 52 (11), 1170-1173 (1981)has described a low viscosity radiator curable formulation comprised ofan unsaturated polyester, an acrylic acid esterstyrene and apolymerization initiator. From the view of the use of stereolithographictechniques, however, this resin formulation has a low green strength andan insufficient photosensitivity.

Also having an insufficient photosensitivity from the view point ofproduction technology are the resin formulations disclosed in U.S. Pat.No. 4,100,141. A low photosensitivity means that long exposure periodsand durations of manufacture are required for making the shaped body.Correspondingly, the photosensitivity of the stereolithographic resinformulation must be so set that from the ratio of the penetration depthof the laser beam into the low viscosity radiation curable resolutionand the radiation energy that is provided, that with low radiationenergy, the greatest possibly curing depth and simultaneously thehighest degree of polymerization can be realized to achieve a good greenstrength and sufficient stability of the resin. formulation by contrastwith auto polymerization. Liquid, radiation curable resin formulationsthat can satisfy the above-mentioned requirements partly are describedfor example in U.S. Pat. No. 5,476,748 or WO 99/50711. Theseformulations which can be referred to as a “hybrid system” contain acombination of radical polymerizable and cationically polymerizablecomponents. They comprise,

firstly, a liquid difunctional or polyfunctional epoxy component or amixture of difunctional or higher functional epoxy components;

secondly, they comprise a cationic photo initiator or a mixture ofcationic photo initiators;

thirdly, they comprise a photoinitiator or a mixture of photo initiatorsfor the free radical polymerization and at least one low viscositypoly(meth)acrylate with a (meth)acrylate functionality of n>2, at leastone diacrylate and a polyol component of the group of hydroxylterminated polyethers, polyesters and polyurethanes.

The artisan is aware that such formulations from the point of view oftoxicology may not be suitable for the production of medicinal productsor can be used only in a limited way for the production of products usedin medicine. For example, it is known that cationic photo initiators cangive rise to skin irritation and allergic reactions. Similarly,compounds with epoxide functions in such formulations may be toxic. Theskilled worker in the art is also aware that acrylate compounds alsohave an increased potential to cause allergic reactions and thus, forexample, those which have been described in EP 0 425 441, EP 0 914 242and EP 0 579 503, because of biocompatibility problems cannot be usedfor the production of earpieces. For use in medicinal technology,monomeric or oligomeric dimethacrylates on the basis of bisphenol-A orbisphenol-F and urethane methacrylate with a functionality of n>2 havebeen found to be effective. By comparison to the acrylate compoundgroup, the methacrylate compound group however has a reduced reactivityfor the stereolithographic technique. As a consequence, the abovedescribed drawbacks with respect to penetration depth of the laser beamand green strength of the prehardened objects can result. Because of thereduced reactivity of this class of compounds, it is necessary, further,to use higher concentrations of one or more photo initiators for thefree radical polymerization. This gives rise to a reduced stabilityagainst autopolymerization of the resin composition.

The skilled worker in the art is also aware that in thestereolithographic technique for the production of a large number ofsmall objects of limited mass, an increased mechanical and thermalloading of the stereolithographic resin formulation can arise which canlead to an auto polymerization of the. stereolithographic resin or to avariation in the characteristics of the resin composition and the shapedbodies to be produced therefrom.

With a reduced resin consumption, the prehardened shaped bodies whichare fixed on a plate from must be removed relatively frequently from thestereolithographic unit. As a result, temperature fluctuations arise inthe stereolithographic resin in the apparatus chamber. Moreover, in theproduction of earpieces, the shaped bodies which are produced have arelatively large surface area to volume ratio. The skilled worked knowsthat the free radical polymerization can produce an inhibition layer onthe surface of the shaped body as a result of oxygen entry. The resinwhich is only polymerized incompletely or is unpolymerized can thusrelease during the shaping process from the surface of the sample holderfor the stereolithographic resin.

For these reasons the stabilization of the low viscosity radiationcurable resin composition is a significant parameter in the productionof earpieces with the stereolithographic technique from the point ofview of production technology. It is therefore desirable that the laserbeam have a penetration depth that is as constant as possible for thecritical energy for the stereolithographic formulation.

The object of the present invention is to make available astereolithographic resin formulation for the production of medicinalproducts, especially earpieces, which will satisfy the mechanical,toxicological and rapid manufacturing requirements forstereolithography.

It has been found that a low viscosity resin mixture of one or moremutually different monomeric or oligomeric dimethacrylates on the basisof bisphenol A or bisphenol F and that, in addition, contains a urethanemethacrylate of a functionality of n<4 and a viscosity <10 Pa s and, inaddition, a monomeric aliphatic or cycloaliphatic dimethacrylate with aviscosity of <3 Pa s and 2,2,6,6-tetramethylpiperidine-1-yloxy (freeradical) can be used for stereolithographic techniques and will yieldupon curing a shaped body prehardened or procured by means of laserwhich has a high green strength. Surprisingly, it has been found furtherthat stereolithographic resin formulations of low viscosity and whichare radiation curable can be produced containing the inhibitor 2,2,6,6-tetramethyl piperidine-1-yloxy (free radical) with a ratio of thecritical energy to penetration depth that can be set within a widerange. The shaped body resulting from full curing or full hardening hasapart from good mechanical properties, excellent biocompatibility, ishard elastic and can be after treated by, for example, grinding orpainting or lacquering.

The subject of the present invention is, therefore, a low viscosityradiation curable stereolithographic resin containing

(a) 55 to 95. weight percent of a monomeric or oligomeric dimethacrylateon the basis bisphenol A or bisphenol F;

(b) 0 to 20 weight percent of a urethane methacrylate with afunctionality n<4 and a viscosity <10 Pa s;

(c) 2 to 15 weight percent of a monomeric aliphatic or cycloaliphaticdiacrylate with a viscosity <3 Pa s;

(d) 0 to 15 weight percent of a monofunctional methacrylate with aviscosity <3 Pa s;

(e) 0.5 to 6 weight percent of one or photoinhibitor a combination of anumber of photo initiators whose absorption lies in the wavelength rangeof the laser beam used;

(f) 0.0001 to 2 weight percent of the inhibitor2,2,6,6-tetramethylpiperidine-1-yloxy (free radical) alone or incombination with inhibitors known in the art;

(g) 0 to 40 weight percent of fillers; 15 (h) 0 to 5% weight percent ofpigments or coloring agents; and

(i) 0 to 5 weight percent of the usual additives like UV stabilizers orflow additives whereby the proportion of the components (a) through (h)together amounts to 100 weight percent.

Preferably, the mixture according to the invention contains:

(a) 60 to 90% by weight or an n-fold ethoxylated bisphenol Adimethacrylate with a degree of ethoxylation of n<10 or a mixture ofn-fold ethoxylated bisphenol A dimethacrylates with a degree ofesterylation of n<10;

(b) 5 to 17 % by weight of an aliphatic or cycloaliphatic urethanemethacrylate with a functionality of n<4 and a viscosity of <10 Pa s;

(c) 3 to 10% by weight of a monomeric aliphatic or cycloaliphaticdimethacrylates with a viscosity of <3 Pa s;

(d) 2 to 10 % by weight of a monofunctional methacrylate with aviscosity <3 Pa s;

(e) 1 to 4% by weight of a photoinitiator or a combination of aplurality of photo initiator whose absorption lies in the wavelengthregion in which the laser beam used lies;

(f) 0.005 to 0.05% by weight of the initiator2,2,6,6-tetramethylpiperidine-1-yloxy (free radical) which may be incombination with the inhibitors known in the art of stereolithography;

(g) 0 to 20% by weight of fillers;

(h) 0 to 5% by weight of coloring agents or pigments; and

(i) 0.01 to 3% by weight of the usual additives like UV stabilizers orflow additives, whereby the proportion of the components (a) to (h)together make up 100 weight percent.

The suitable compounds of the component (a) are, for example, thedimethacrylates of (n)-alkoxylated bisphenol-A likebisphenol-A-ethoxylate(2)dimethacrylate, bisphenol-A-ethoxylate (4)dimethacrylate, bisphenol-A-propoxylate (2)dimethacrylate,bisphenol-A-propoxylate-(4)-dimethacrylates, as well as dimethacrylatesof the (n)-alkoxylated bisphenol-F likebisphenol-F-ethoxylate(2)-dimethacrylate,bisphenol-F-ethoxylate(4)dimethacrylate, bisphenol-F-propxylate (2),dimethacrylate, bisphenol-F-propoxylate(4)dimethacrylate, and mixturesthereof. Preferably one uses monomeric or oligomeric dimethacrylates ofa bisphenol-A basis and especially thebisphenol-A-ethoxylate(2)dimethacrylate and thebisphenol-A(4)dimethacrylate. Especially preferred are mixtures of thesetwo dimethacrylate on a bisphenol-A basis with a proportion of thebisphenol-A ethoxylate(4)dimethacrylate>than that of thebisphenol-A-ethoxylate(2)dimethacrylate.

The urethane methacrylates used in the formulation according to theinvention as component (b) with a functionality<4 are known to theskilled worker in the art and can be produced in a known manner byreacting for example a hydroxyl terminated polyurethane with methacrylicacid to the corresponding urethane methacrylate or by reacting anisocyanate prepolymer with hydroxy methacrylates. Corresponding methodsare known for example from EP 0 579 503. Urethane methacrylates are alsoavailable commercially and are marketed for example under thedesignation PC-Cure® by the Firm Piccadilly Chemicals, under the productdesignation CN 1963 by the Firm Sartomer and under the designationGenomer® by the Firm Rahn.

Preferably the urethane methacrylate which is used which has afunctionality of n<4 and is produced from aliphatic educts, areespecially the HEMA and TMDI containing isomer mixture 7,7,9- (or7,9,9-)trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-1,16-diol-dimethacrylate.

Examples of the compounds of component (c) are:1,3-butanedioldimethacrylate, 1,6-hexanedioldimethacrylate,1,3-butleneglycoldimethacrylate, diethyleneglycoldimethacrylate,ethyleneglycoldimethacrylate, neopentyldimethacrylate.polyethyleneglycoldimethacrylate, triethyleneglycoldimethacrylate andtetraethyleneglycoldimethacrylate and preferably 1,4-butanedioldimethacrylate. Such products are commercially available andcan be obtained, for example from the Firm Sartomer.

The component (d) of the formulation according to the invention can forexample include the following compounds:

allylmethacrylate; methyl-, ethyl, n-propyl, n-butyl, isobutyl, n-hexyl,2-ethylhexyl, n-octyl, n-decyl, n-dodecyl, isobornyl, isodecyl, lauryl-and stearylmethacrylate: 2-Hydroxyethyl-, 2- and 3-hydroxypropylmethacrylate; tetrahydrafurfurylmethacrylate andcyclohexylmethacrylate. Especially preferred is the ethylmethacrylate.

As the component (e) photo inhibitors or all types can be used whichform free radicals with corresponding irradiation, these include knownphotoinitiator compounds including benzoins, benzoinethers, likebenzoine, benzoinmethether, benzoinethylether, benzoineisopropylether,benzoinephenylether and benzoineacetate, acetophenones likeacetophenones, 2,2-dimethoxy acetophenone and 1,1 dichlor acetophenone,benzil, benzylketals like benzyldimethylketal and diethylketal,anthraquinones, like 2-methylanthraquinone, 2-ethylanthraquinone,2-tertiarybutylanthraquinones, 1-chlorobutylanthraquinones. 202-amylanthraquinones, triphenylphosphine, benzoylephosphino like, forexample, 2,4,6-Trimethylbenzoyldiphenylphosphinoxide (LUZIRIN TPO) andbis(2,4,6-trimethylbenzoylphenylphosphinoxide, benzophenones, likebenzophenone and 4,4-bis(n,n′-diethylamine)-benzophinone, thioxanthoneand xanthone, acridine derivatives, phenazine derivatives, quinoxalinederivatives or 1-phenyl-1,2-propanedione-2-0-benzoyloxime,1-aminophenylketone or 1-hydroxyphenylketone or1-hydroxycyclohexylphenyl ketone, phenyl 1-hydroxyisopropyl)ketone andisoproplophenyl-(1-hydroxyisopropyl)-ketone.

Especially preferred compounds which can be used in combination with aNd:YVO₄ solid body laser, arebis-(2,4,6,-trimethylbenzoyl)-phenylphosphineoxide2,4,6-trimethylbenzoyl-diphenylphosphinoxide,2-hydroxy-2-methylpropiophenone, hydroxy cyclohexyl phenyl ketone andmixtures of these photo initiators.

Into the mixture according to the invention,2,2,6,6-tetramethylpiperdine-1-yloxy (free radical) is added. Theinfluence of the concentration of the2,2,6,6-tetramethylpiperdine-1-yloxy (free radical) in the exemplaryformalation 1 upon the penetration depth of the laser beam and energyhas been shown in FIGS. 1 and 2. The laser penetration depth and thecritical energy are determined by means of the window pane methoddescribed by P. F. Jacobs and D. R. Read in Rapid Prototyping andManufacturing of the Society of Manufacturing Engineers, Dearborn, Mich.(1992), 1^(st), ed., pgs. 263-280. From the figures, it will be apparentthat even limited addition of the 2,2,6,6-tetramethylpiperdine-1-yloxy(free radical) gives rise to a surprising increase in the criticalenergy with only a limited change in the penetration depth of the laserbeam. This influence of the inhibitor2,2,6,6-tetramethylpiperdine-1-yloxy (free radical) upon the relevantparameters Ec (critical energy in mJ cm⁻²) and Dp (laser penetrationdepth in mils) of the stereolithographic resin, the effect differingfrom the influence of the inhibitors like, for example,hydroquinone-monomethylether which are added to stereolithographicresins in accordance with the state of the art. The influence of thehydroquinonemonomethylether on the parameters Ec and Dp in the exemplaryformulations 2 is shown in FIGS. 3 and 4.

By comparison with the effect of the2,2,6,6-tetramethyl-piperdine-1-yloxy (free radical) the critical energyof the stereolithographic resin can only be adjusted by significantlyhigher concentrations of hydroauinonemonomethylether within asignificantly smaller frame. Moreover, the effect of thehydroquinonemonoether methyl inhibitor upon the laser penetration depthis greater than in the case of the use of the 2,2,6,6-tetramethylpiperdine-1-yloxy (free radical). This is especially disadvantageouswhen the parameters of colored or opaque stereolithographic resins mustbe adjusted since the addition of pigments or coloring agents alsoreduce the laser penetration depth. The above mentioned approach fromthe point of view of productivity can lead to undesired increases in thetime for producing the shaped bodies. In the case of stereolithographicresins in the medicinal field and especially in the case ofstereolithographic resins for the production of earpieces, colored oropaque stereolithographic resin formulations are especially relevant.

EXAMPLE 1

70.3- x weight percent bisphenol A-ethoxylate(4)dimethacrylate.

14.4 weight percent bisphenol-A-ethoxylate-(2)-dimethacrylate.

9.2 weight percent 7,7,9-(or7,9,9)trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecane-1,16-diol-dimethacrylate.

4.6 weight percent 1,4-butanedioldimethacrylate.

1.5 weight percent bis(2,4,6-trimethylbenzoyl)-phenylphosphinoxide.

X weight percent 2,2,6,6-tetramethylpiperdine-1-yloxy (free radical).

The 2,2,6,6-tetramethylpiperdine-1-yloxy (free radical) is used inconcentrations of x=0; 0.005; 0.01; 0.02 and 0.05 weight percent.

EXAMPLE 2

70.3-X weight percent bisphenol-A-ethoxylate(4)dimethacrylate.

14.4 weight percent bisphenol-A-ethoxylate(2)dimethacrylate.

9.2 weight percent 7,7,9-(or7,9,9)trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecane-1,16-diol-dimethacrylate.

4.6 weight percent 1,4-butanedioldimethacrylate.

1.5 weight percent bis(2,4,6-trimethylbenzoyl)-phenylphosphinoxide.

X weight percent 2,2,6,6-tetramethylpiperdine-1-yloxy (free radical).

The inhibitor hydromethylether was used in concentrations of X=0; 0.1;0.2; 0.4; 0.6; 1; and 2 weight percent.

FIG. 1: The influence of the concentration of2,2,6,6-tetramethylpiperdine-1-yloxy on the laser penetration depth Dpof the resin formulations of Example 1.

FIG. 2: The influence of the concentration of2,2,6,6-tetramethylpiperdine-1-yloxy on the critical energy of the resinformulations of Example 1.

FIG. 3: The influence of the concentration ofhydroquinonemonomethylether on the laser penetration Dp of the resinformulation of Example 2.

FIG. 4: The influence of the concentration ofhydroquinonemonomethylether on the critical energy of the resinformulations of Example 2.

The mixtures of the invention can, if required, have other additivessupplied to them, for example flow agents, UV stabilizers, wettingagents, fillers, coloring agents and pigments. In the sense of theinvention, especially suitable coloring agents are anthroquinonediestuff preparations like those marketed for example by Bayer under thename Macrolex.

Use Examples:

EXAMPLE 3 A Yellowish Opaque Stereolithographic Resin 66.69%bisphenol-A-ethoxylate(4) dimethacrylate

15.6 weight percent bisphenol-A-ethoxylate-(2)-dimethacrylate.

weight percent 7,7,9-(or7,9,9)-trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecane-1,16-diol-dimethacrylate.

5 weight percent 1,4-butanedioldimethacrylate.

1.5 weight percent bis(2,4,6-trimethylbenzoyl)-phenylphosphinoxide.

0.8 weight percent pyrogenic silica.

0.2 weight percent iron oxide pigment.

0.01 weight percent 2,2,6,6-tetramethyl piperdine-1-yloxy (freeradical).

EXAMPLE 4 A Blue Transparent Stereolithographic Resin 69.035% bisphenolA-oxylate(4) dimethacrylate

15.6 weight percent bisphenol A ethacrlyate (2) dimethacrylate.

10 weight percent 7,7,9- (or 7,9,9)trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecane-1,16-diol-dimethacrylate.

3.8 weight percent 1,4-butane diol dimethacrylate.

1.5 weight percent bis(2,4,6-trimethylbenzoyl)-phenylphosphinoxide.

0.3 weight percent anthroquinone dyepreparation (containing CI solventblue 97).

0.025 weight percent UV stabilizer.

0.01 weight percent 2,2,6,6-tetramethylpiperdine-1-yloxy (free radical).

EXAMPLE 5 A Red Transparent Stereolithographic Resin 69.055%bisphenol-A-ethoxylate(4)dimethacrylate

15.6 weight percent bisphenol-A-ethoxylate(2)dimethacrylate.

10 weight percent 7,7,9-(or7,9,9)trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecane-1,16-diol-dimethacrylate.

3.8 weight percent 1,4-butanedioldimethacrylate.

1.5 weight percent bis(2,4,6-trimethylbenzoyl)-phenylphosphinoxide.

0.8 weight percent pyrogenic silica.

0.2 weight percent iron oxide pigment.

0.01 weight percent 2,2,6,6-tetramethylpiperdine-1-yloxy (free radical).

The relevant parameters for the use of the above mentioned resins instereolithography are collected in Table 1. All viscosity measurementswere carried out at 23° C. with a CVO 120 rheometer of BohlinInstruments. The determination of the bend strength, the modulus ofelasticity and the elongation were carried out in accordance with EN ISO178 (1996) with a Zwick 1- test machine of the Zwick firm. For thedetermination of the Ec and Dp values by means of the above mentionedwindow pane method, the average value from 10 window pane sample bodieswas determined. The sample bodies were produced with a Viper Si² SLAapparatus equipped with an ND:YVO₄ solid body laser. The green bodieswere after hardened with a stroboscope lighting unit Sonolux PR fromInnovation Meditech by means of 4000 flashes.

With the hardened sample bodies of the stereolithographic formulationsfrom Examples 3 to 5, studies were made of the cyclotoxic, irritationand allergic reaction or sensitivity potential of the formulations.

The determination of the irritating effect was carried otu based uponISO 10993-10 (2002), ISO 10993-12 (2002) and ISO 17025 (1999) tests onwhite New Zealand rabbits. These were injected with control solutionsand test extracts from the sample bodies intracutaneously. From thetests the above mentioned stereolithographic formulations were found tobe negligibly irritating. In a further test series for determining thebiological reaction of the mamalian cell culture (L929) to the samplebodies, elution tests according to ISO 1099-35 (1999), ISO 10993-12(2002) and ISO 17025 (1999) were carried out. No biological reactivity(00) was found with the L929 mamalian cells. Correspondingly, the abovementioned materials were nonocytotoxic and satisfied the requirements ofISO 1099-395.

In addition, the allergic potential and the sensitizationcharacteristics of the sample bodies were evaluated in a test series onthe basis of ISO 10993-10 (2002) and ISO 1093-12 (2002) in accordancewith F. N. Marzulli and H. I. Maibach (eds.), 5^(th) ed., 1996, pages403, 440-441, Hemisphere Publishing Corporation, New York, B. Magnussonand A. M. Kligman, J. Invest. Dermatol. 52:268-176 (1969); B. Magnussonand A. M. Kligman, 1970, Allergic Contact Dermatitis in the Guinea Pig,Identification of Contact Allergens, Springfield, Ill., Thomas. Theabove mentioned stereolithographic formulations induced no biologicalreaction (0° sensitisation. In accordance with the Kligman scale therewas a reaction of the first degree so that the allergizing potential ofthe stereolithographic formulation is considered to be weak utilizingthe standard staging. A sensitization rate of the first degree inaccordance with Magnusson and Kligman is considered as not significant.

From these results, it is clear that the stereolithographic formulationsaccording to the invention can be used in medicinal technology,especially for the production of ear pieces.

In Table 3 the mechanical and chemical parameters of the commerciallyavailable opaque stereolithographic resin (7,7, LS7410) of the VanticoFirm, which was conceived for use in medicinal technology, have beengiven. From the tables, one can see that by comparison to theformulation according to the invention, there is advantageously asignificantly reduced viscosity and higher Dp value. In addition, themechanical properties of bending strength and modulus of elasticity ofthe prior art of the disclosed formulation are significantly above thevalues for the commercially available produced. In addition, it can beseen that the Vantico material cannot be treated as in the cytotoxicstage but in a test series for determining cytotoxicity a slightbiological reaction of the first degree can be induced. From the pointof view of use in the field of medicinal technology, this is not thecase with the formulations according to the invention.

All of the serial lithographic resin formulations according to theinvention in Examples 3 to 5 have, from the point of view of productiontechnology, an ideal viscosity, 2250 mpa s and green strength.

The conventional technique for producing earpieces based upon autopolymerizing or light hardening materials require shaping the earpiecein a negative mold. By comparison to the materials used conventionallyfor producing the earpieces (for example Fotoplast® of the DreveOtoplsatic GmbH Firm (see Table 3) the mechanical values of bendingstrength and elastic modulus of the completely hardened molded bodiesare significantly higher. TABLE 1 Parameters of the StereolithographicResin Formulations of Examples 3 to 5 Composition CompositionComposition Property Ex. 3 Ex. 4 Ex. 5 Viscosity at 23° C., Mpa s 700860 820 E-Modulus-Green, N mm⁻² 1544 750 862 Bending Strength Green, 8148 51 N mm⁻² Elongation Green, % 14 14 15 E-Modulus hardened shapedbody, N mm⁻² Bending strength of hardened 120 115 98 shaped body, N mm⁻²Elongation of hardened 8 8 9 shaped body, % Ec, mJ cm⁻² 14.4 23.4 32.1Dp, mils 4.1 4.3 5.6

TABLE 2 Data as to Commercially Available Stereolithographic Resin ofthe Firm Vantico Vantico Stereolithographic Property Resin Viscosity at23° C., Mpa s 2350 Elastic Modulus of hardened shaped body, N mm⁻² 1612Bending strength of hardened shaped body, N, mm⁻² 81 Elongation ofhardened shaped body. % 12 Ec, mJ cm⁻² 3.4 Dp, mils 6.6

TABLE 3 Mechanical values of commercially available products for theproduction of earpieces E-modulus, Bending Elongation Material N mm⁴strength to Break Fotoplast S/IO Blue transparent, 1513 81 10 Lot.201504 Fotoplast S/IO Red transparent, 1527 84 13 Lot. 301531 FotoplastS/IO Colorless transparent, 1602 88 11 Lot. 203523 Fotoplast S/IO Browntransparent, 1374 74 14 Lot. 209540

1. A biocompatible, low viscosity, radiation curable formulation,especially for stereo, for use in medicinal technology, especially forproducing earpieces, comprising: a) 55 to 95 weight percent of amonomeric or oligomeric dimethacrylate on the basis of bisphenol-A orbisphenol-F; b) 0 to 20 weight percent of a urethane methacrylate with aviscosity>4 and a viscosity<15 Pa s; c) 2 to 15 weight percent of amonomeric or aliphatic or cycloaliphatic dimethacrylate with aviscosity<5 Pa s; d) 0 to 15 weight percent of a monofunctibnalmethacrylate with a viscosity<3 Pa s; e) 0.5 to 6 weight percent of oneor a combination of photoinitiators whose absorption lies in thewavelength range of the laser beam used; f) 0.001 to 2 weight percent ofthe inhibitor 2,2,6,6-tetramethylpiperdine-1-yloxy (free radical) whichcan be present in combination with known inhibitors; g) 0 to 40 weightpercent of fillers; h) 0 to 5 weight percent of color pigments; i) 0 to5 weight percent of usual additives like UV stabilizers or flowadditives, whereby the proportion of the components a to h togetheramounts to 100%.
 2. The formulation according to claim 1 comprising: a)60 to 90 weight percent of an n-fold ethoxylatedbisphenol-A-dimethacrylate with a degree of ethyloxilation of n<10 or amixture of n-fold ethoxylated bisphenol-A-dimethacrylate with a degreeof n<10; b) 5 to 17 weight percent of an aliphatic or cycloaliphaticurethane methacrylate with sensitivity of n<4 and a viscosity of <10 Pas; c) 3 to 10 weight percent of an aliphatic or cyclo-aliphatic urethanedimethacrylate with and a viscosity<3 Pa s; d) 2 to 10 weight percent ofa monofunctional methacrylate with a viscosity<3 Pa s; e) 1 to 4 weightpercent of one or a combination of a plurality of photoinitiators whoseabsorption is in the wavelength range of the laser beam used; f) 0.005to 0.05 weight percent of the initiator2,2,6,6-tetramethylpiperdine-1-yloxy (free radical) optionally incombination with known inhibitors; g) 0.20 weight percent of fillers; h)0 to 5 weight percent of color pigments; i) 0.01 to 3 weight percent ofconventional additives like UV stabilizers or flow additives whereby theproportion of the components of (a) to (h) amount together to 100%.